Resin compositions containing acrylic copolymers each using an alkyl(meth)acrylate as a monomer component have been conventionally of ten used as pressure-sensitive adhesives in general adhesive sheets or labels. In recent adhesive sheets or labels for electronic parts, an alcohol component derived from a side chain-decomposed product of an acrylic copolymer serves as outgases, and the outgases may cause problems in the operation of the electronic parts. Accordingly, such contrivance as described below is needed. Specifically, there is used a monomer component whose portion serving as a side chain in the acrylic copolymer has a low molecular weight, or there is not used the unsaturated carboxylic acids such as a (meth)acrylic acid that aid the decomposition of the portion serving as a side chain (which may hereinafter be referred to as “(meth)acrylic acid and the like”).
In addition, adherends for the adhesive sheets for electronic parts are mainly metals, and the adhesive sheets are used under various environments. Accordingly, the adhesive sheets are each requested to have such performance as to be free of any corrosive action on the metals. The following condition is particularly important in imparting the performance. None of a (meth)acrylic acid and the like responsible for corrosion is used as a monomer.
Meanwhile, carboxyl groups serving as side chains of the acrylic copolymers derived from a (meth)acrylic acid and the like each having high polarity each play an extremely important role in an adhesion for a metal. Accordingly, mere avoidance of the use of a (meth)acrylic acid and the like causes another problem. In other words, in the pressure-sensitive adhesives formed of the resin compositions mainly formed of the acrylic copolymers that do not use a (meth)acrylic acid and the like as their monomers, no improvement in polarity of the entirety of each of the polymers by virtue of a (meth)acrylic acid and the like can be obtained, and an adhesion for a metal adherend is not sufficient. As a result, blisters, release, and the like may occur in the adhesive sheets.
In addition, it has been known that assorted cross-linking promoters are used upon a cross-linking process with an isocyanate-based cross-linking agent. However, the cross-linking promoters are unsuitable for being adhered to electronic parts because of the following reasons. An organotin-based cross-linking promoter (Patent Document 1) is not suitable for use in the application because organotin itself serves as outgases to contaminate an electronic part as well as silicone.
A cross-linking promoter such as an amino group-containing compound (Patent Document 2) has so low a molecular weight as to volatilize at the time of drying to weaken its effect. In addition, it is anxious that the cross-linking promoter may be detected as an outgas component.
In addition, an amino group-containing compound having a high molecular weight is not suitable for the application either because the application of the compound may contaminate an adherend.
Further, it has been proposed that a nitrogen atom-containing copolymerizable monomer such as an amide group-containing monomer which is a high-polarity monomer is used as a monomer component for any one of the acrylic copolymers (Patent Document 3). However, the monomer remarkably increases a Tg of the copolymer, and hence a reduction in wettability occurs. As a result, it causes a problem that a sufficient adhesion cannot be obtained.
In addition, it has been proposed that cross-linking promoters including organozinc compounds such as zinc naphthenate and zinc 2-ethylhexanoate, and organolead compounds such as lead stearate, lead naphthenate, and lead 2-ethylhexanoate are used (Patent Document 4). However, the use of such the cross-linking promoters causes a problem that heavy metals baneful to the human body have to be used.    Patent Document 1: JP 2000-44896 A    Patent Document 2: JP 2005-154531 A    Patent Document 3: JP 2005-325250 A    Patent Document 4: JP 2006-96956 A